1. Field of the Invention
This invention relates to the drive mechanism for a scanner.
2. Background of the Invention
A scanner is a common computer peripheral equipment which uses an internal image pick-up device to pick up document images and to transfer the pick-up image into digital signals as input to a computer which then performs image processing of the document.
FIG. 1A is a diagram showing a conventional scanner 1. The conventional scanner 1 comprises a housing 12 with an upper opening 11, a document 13 to be scanned, and an image pick-up device 14. The document 13 can be placed above the upper opening 11 of the housing 12, and the image pick-up device 14 scans the image by a drive mechanism (not shown) provided by the scanner 1, with the image pick-up device 14 being an optical path device of Charged Couple Device (CCD) or Contact Image Sensor (CIS).
FIG. 1B illustrates the drive mechanism 2 of the conventional scanner 1. The drive mechanism 2 is mounted in the housing 12 and positioned below the document 13, and comprises a scanning path 21, a delivery device 22, and a set of decelerated gears and a motor 24.
The scanning path 21 consists of a slip bar 211 and a sliding rail 212, with the slip bar 211 and the sliding rail 212 parallel each other, and with the image pick-up device 14 having a guide hole 141 and rotating wheel 142 that cooperate with slip bar 211 and sliding rail 212, respectively, so as to scan back and forth in the scan path 21.
The delivery device 22 comprises an actuating gear 221, a passive gear 222 and an annular gear belt 223, with the actuating gear 221 and the passive gear 222 mounted in the two ends of the housing 12 respectively, the annular gear belt 223 being set on the two gears 221, 222 respectively and parallel with scan path 21, and with the annular gear belt 223 combining with the image pick-up device 14.
A reduction gear set 23 is combined with the motor 24 and the actuating gear 221, and its primary function is to provide appropriate reduction ratio so that rotational speed (torque) of the motor 24 is reduced to an appropriate extent and then to communicate motive power to the actuating gear 221.
Thus the scanning mode of the drive mechanism 2 is to promote the rotation of the delivery device 22 by way of output power of the motor 24 so that the annular gear belt 223 drives the image pick-up device 14 to move back and forth in the scan path 21 so as to scan the document 13.
However there are many design problems with the conventional scanner 1. Since current consumers not only demand to increase the resolution of the scanner 1, they also demand that scanning speed be as fast as possible. However, if the resolution of the scanner 1 is increased, the speed of image processing becomes slow, which then requires adjusting the reduction ratio of the reduction gear set 23 or adopting higher rotational speed of the motor 24 so as to increase the scanning speed of the scanner 1.
Unfortunately, all motors, whether DC motor or stepping motor, have defined specifications and rotational speed (torque) range limits, so little can be done to increase the rotational speed of the motor 24. Hence increase in the speed of image processing is typically achieved by changing the reduction ratio of the reduction gear set 23. However, even changing the reduction ratio of the reduction gear set 23 may not be enough.
It is an objective of the present invention to provide a drive mechanism for a scanner. The drive mechanism provides wider scope of scanning speed for a scanner and is matched with the scanning speed design of the scanner that has various resolutions.
The drive mechanism according to the present invention comprises a delivery device, two passive skew gears, a first rotational speed motor and a second rotational speed motor. The delivery device consists of an actuating wheel, a passive wheel and an annular conveyor belt, with the conveyor belt carried by the actuating wheel and the passive wheel. The axes of the two passive skew gears are attached to two ends of the inside diameter of the actuating wheel. The output shaft of the first rotational speed motor is provided with an actuating skew gear that engages the two passive skew gears. The output shaft of the second rotational speed motor is provided with an actuating skew gear that engages the two passive skew gears. The drive mechanism uses the two motors to drive the two passive skew gears so as to better control the rotational speed and to scan the document image.
The two passive skew gears and two actuating wheels of two motors form a differential gear. Because the axes of two passive skew gears are attached to two ends of the inside diameter of the actuating wheel, averaging the sum of the first motor rotational speed and the second motor rotational speed will provide the rotational speed of the actuating wheel. Hence the motors of two different rotational speeds are used to mutually coordinate and obtain a particular or more appropriate rotational speed value for the actuating wheel.